Electrical musical instrument



, March 17, 1942. HAMMOND ELECTRICAL MUSICAL INSTRUMEZIT Filed April 2, 1938 3 Sheets-Sheet 1 fnve/vfor y Laure/7s Ham/770% I haw 64 MM Affoi/ eys NOTE MO. 1

NOTE NO. 6/

March 17, 19420 L HAMMOND ELECTRICAL MUSICAL INSTRUMENT Filed April 2, 1938 3 Sheets-Sheet 2 I'm/enrol" Laurens Hammond B9 MwiM L. HAMMOND ELECTRICAL MUSICAL INSTRUMENT March 17, 1942.

Filed April 2, 1938 3 Sheets-Sheet 5 rd m m 8 V m m 5 w I U a L 5 WELL CONTROL PRE -A MPLlF/EE Afforneys POWER AMPLIFIER LOUD SPEAKER Patented Mar. 17, 1942 7 UNITED STATES PATENT OFFICE 2,276,389 ELECTRICAL MUSICAL INSTRUMENT Laurens Hammond, Chicago, Ill. Application April 2, 1938, Serial No. 199,612 15 Claims. (Ci. iii-1.25)

My invention relates generally to electrical musical instruments, and more particularly to a system for generating electrical impulses of the frequencies of the musical scale by purely electrical means.

It has been proposed in the past to obtain the electrical impulses of the various frequencies required for an electrical musical instrument from oscillators, one for each note of the scale. Generally speaking, such systems have not proved satisfactory due to the difficulty of maintaining the oscillators exactly in tune with one another. In an instrument of the piano type, it is necessary to provide 88 such oscillators, and the dif ficulty of tuning such a large number of oscillators is very great, when the fact is taken into consideration that the frequency of oscillation of the oscillation generators may change considerably with the slight changes in the characteristics of the tubes and other circuit elements due to aging and to atmospheric conditions as well as to changes in the voltage of the power supply.

Of course oscillators may be made more-stable, with reference to the frequency of oscillation, by various means, but such means usually add materially to the complication and cost of the oscillator, and since in a piano type of instrument it would be necessary to provide 88 such oscillators. the complexity and cost of the system is multiplied so as to render impractical a system in which an individual and independently tunable oscillator is employed for each note.

To overcome these defects and disadvantages, I have provided a system employing twelv master oscillators, one for each of the notes of the highest octave of the instrument. Since there are only twelve of these master oscillators, it is practical to make them of good quality so that their frequencies will be very stable. Each of the twelve master oscillators is connected in cascade with a series of relaxation oscillators, one for each of the corresponding octave notes in the lower octaves. For example, the master oscillator for the generation of the frequency required for the note A of the highest octave will be connected to the relaxation oscillator for the note A of the second highest octave, and the latter will be connected to the relaxation oscillator for the note A in the third highest octave, etc. Thus, all of the oscillators for the notes A of the instrument will be connected in a cascaded series and each oscillator in the series will have its frequency stabilized by the oscillator for the note which is one octave higher and which therefore generates a frequency twice that of the oscillator which is controlled thereby.

In a system of this character, a variation in the frequency of oscillation of the master oscillator will cause a corresponding variation in the frequencies of oscillation of each of the other oscillators which is connected in cascaded selies with such master oscillator. For example, the master oscillator for the generation of a frequency of the note A of the highest octave will, if tuned to the pitch A440, oscillate at a fre quency of 3520 C. P. S. If the tuning of this master oscillator is changed so that instead of oscillating at 3520 it oscillates at 3522 C. P. S., the relaxation oscillator for the generation of the note A of the second highest octav will be controlled by the master oscillator so as to generate a frequency exactly one half of th frequency of the master oscillator, namely, 1761 C, P, S. Similarly, the frequencies generated by the oscillators for the notes A in the lower octaves will each be proportionately increased. It is therefore possible, by accurately tuning the twelve master oscillators, to tune all of the remaining oscillators.

Furthermore, if the frequency of oscillation of the master oscillator is periodically shifted at a vibrato frequency rate, the frequencies of 0scillation of the relaxation oscillators controlled by such master oscillator will be shifted correspondingly. Thus it is possible to provide a very simple and effective means to produc vibrato effects in an instrument employing the generating system of my invention.

Inasmuch as the subject of the causes and effects of vibrato in a musical instrument are hot widely known, the underlying principles of vibrato in a musical tone will be briefly discussed.

Strictly speaking, a vibrato is the musical result obtained by periodically changing th frequency of the tone being sounded through a relatively narrow range of pitch change, at a periodic rate in the order of 7 C. I, S. The periodic change in frequency is normally not sufficiently'great to give the effect of actual pitch change but rather to give the effect of warmth of tone. The instrument upon which the effect of vibrato is most frequently heard and most easily recognizable is the violin. The vibrato effect is not, however, a feature exclusively of the violin tone. It is present in the tones of many other musical instruments although it is probably not as readily recognized in the tones of other instruments. For example, when chords ar played upon a piano, there is a pronounced of the fundamental. standard piano as may quickly be shown by vibrato, although it is seldom recognized as such by the average listener. This characteristic of the tone of thestandardpiano is resultant from the fact thatthe tone of the piano note, especially in the lower'register, is made up of a fundamental and long series of partials.

A further characteristic of the standard piano lies in the fact that the tone of a piano note, especially in th lower register, is made up of a fundamental and a long series of partials, which partials are not true harmonics of the fundamental-in the sense that they are of frequencies which are not exact integral multiples of the fundamental. -This is due to the fact that whereas the string continues to generate sound over a considerable period of time, it is not continuously excited, but receives all of its energy during the blow of the hammer. If the string were an ideal one-that is to say, having no stifiness of its own, a uniformly distributed mass in a diameter which could be completely neglected by comparison with its length-then it could be shown that such a string would give rise to partials of frequencies which were exact harmonics This is not'the case in the watching the pattern shown on a cathode ray oscillograph operated from a microphone picking up the sound of a piano note. If all the partials of the tone were exact harmonics, the

shape of the pattern would not change appreciably during the decay of the note. Actual experiment will immediately show small amplitude high frequency waves which travel rapidly along on the wave produced by the lower frequencies.

Pianos are usually tuned by a beat system, and

whereas the methods of tuning differ in detail, all of them tend to expand the musicalscale in such a way that the highest notes of the scale are sharp and the lower notes are flat.

Let us assume therefore, that a piano has just been tuned in accordance with the best practice, and assume further, that double octaves are struck, for instance, A's Nos. 13, 25, 37 and 49. The second harmonic of the lowest A (13) is very close in frequency to the fundamental of the A (25) an octave higher. Similarly, the fourth harmonic of the A (13) is of a frequency in the vicinity of the fundamental of the A (3'7) two octaves higher, and in the vicinity of the second harmonic of the A (25) one octave higher. The eighth harmonic of the lowest A (13) is close in frequency to the fundamental of the A (49) three octaves up and close to the fourth harmonic of the A (25) one octave higher, and close to the second harmonic of the A (37) two octaves higher, etc. Now it is not possible to tune these four A's so that there will be no beats between them. Not only is it not possible to do so in a practical sense, but it is theoretically impossible as well. Without therefore considering the effects of the tempered musical scale which introduces beats in chords where notes of other than unison pitches are involved, it will be seen that beats will occur between frequencies which are close to one another and which are derived from the different notes of chords.

Further, beats are introduced for another reason. Except for the very lowest notes of the standard piano, either two or three strings are used for each note. The multiple strings cannot be tuned exactly alike as a practical matter, or even if they weregthey would not remain so for very long after tuning. Chords played on a freshly tuned piano give rise to groups of frequencies listeners.

which are very close to one another but areno't the same. The amounts of their differencesare not of an order to suggest to the listener that the piano is out of tune, but these frequency differences play a very important part in characterizing the general effect and in giving life to the tone. This is due to the fact that when two frequencies which are close to one another are temporarily in phase with one another they reinforce each other, whereas when they are ternporarily out of phase they tend to cancel one another. tinuously changingcertain frequencies coming on while others are going off, etc. These effects are generally similar to the effect of a vibrato on a stringed instrument such as a violin.

In listening to the piano, one is ordinarily not aware of the vibrato effect because of the fact that a vibrato is associated in our minds with a definite simple rhythmical change such as is heard when a violinist rocks his finger on the string, In that case, he changes the effective length of the string in such a way as periodically to raise and lower the pitch. This is a change in frequency which, together with the pattern of the enclosure in which he is playing, produces at the listener's cars a change both in frequency and in intensity.

A simple vibrato on a melody instrument such as the violin occurs at a periodicity which may be counted by a listener. Depending on room conditions and other factors, this may sound very much like a tremulant to the listener, and there is no very clear difference, under some circumstances, between a vibrato, which is customarily thought of as a periodic change in frequency, and a tremulant, which is usually defined as a periodic change in intensity. The effect of the vibrato is enhanced by applying it at different frequencies to different notes, such that when a chord is played no definite rhythmic beating is perceptible. It may be stated in a general way, that when the listener becomes conscious of such rhythm to the extent tat he is aware of the beat itself rather than of the effect of warmth which it gives to the tone, the effect is then unsatisfactory to many He is never able to count the vibrato effects of the piano and has therefore come to think of the piano as an instrument without vibrato, which is not the fact in a technical sense. Very strong vibrato effects are present in the standard piano, but they are of a very complex nature. The beats between the different frequencies do not last long because the notes decay, and there are a great many different rates of beat going on at the same time. To count them it is necessary to focus the attention on one particular beat and to start to count. Time does not permit much in this process, because before the rate is established clearly in the mind, the effect is gone.

In the instrument of my invention, I have a system for continuously shifting the frequencies of each-note in a rhythmical manner. Thus, if a single note is played, the effect may be counted, if the attention is attracted to it, as is the case with the solo violin note. I'provlde a method. however, for putting a diflerent vibrato rate on the difl'erent notes so that when chords are played there are many vibratos of different rates going on at the same time. This produces an effect too complex for the listener to analyze mentally in the sense that he is unable to pick up these complex rhythms before they are gone. The shifting of the frequencies of different notes in a manner in which one is independent of another Thus, the acoustic spectrum is con aaracso produces a musical effect which is highly desirable. The complex tone of a chord, as in the standard piano, is then made up of many groups of frequencies which are shifting in audibility, so that some are growing more prominent while others are becoming less so.

It can be shown that a change in frequency such as is obtained in shifting the frequencies of the oscillators to produce a vibrato effect, ordinarily produces a slight change in the intensity of the note. Similarly, a tremulant device, which is considered primarily as a note intensity varying means ordinarily causes a change in the frequency of the note being sounded. Under certain circumstances there may be very little difference between the musical effect of a tremolo and that of a vibrato. Most musicians agree thatthe more desirable effect is that of the vibrato, or shift in frequency.

If the frequency of a note being sounded is periodically shifted at a vibrato rate, the effect (mathematically as well as musically) is not one particularly of frequency change, but rather the addition to the note of additional frequencies. The frequencies generated must, however, be multiples of the vibrato frequency. For example, if the fundamental frequency of the tone, before the vibrato was applied was 100 cycles, the second harmonic would have a frequency of 200 cycles. If a vibrato frequency of '7 cycles were then impressed upon the note, the resultant frequencies would have to be frequencies which are harmonics of the vibrato frequency of 7 cycles. Thus, the second harmonic, 200 cycles, in being modulated by the vibrato frequency of 7 cycles could not produce frequencies of 193 and 207 because these frequencies are not exact multiples of the vibrato frequency. Instead, the new frequencies produced upon modulation of the second harmonic of the tone by the vibrato frequency would be 196 and 203, the nearest harmonic frequencies of the vibrato frequency of '7 cycles. In addition to the frequencies of 196 and 203 cycles, there would be present in the tone, some energy at the frequencies of 189 cycles and 210 cycles, and other multiples of '7.

The vibrato effect in musical tones obtains its musical allure by virtue of the fact that by the introduction of a vibrato frequency a large number of frequencies are necessarily developed because of the modulation of the original tone frequency wave by. vibrato frequency wave- The acoustical energy of the note being sounded will thus be distributed over a wide range of frequencies. Generally speaking, the human ear prefers such musical sounds in which the energy is widely distributed, to sounds in which the energy is concentrated at a single frequency or a limited number of frequencies. Tones in which the energy is distributed over a wide range have become recognized as the more beautiful, full and rich tones.

In the system of generating frequencies 'for musical instruments shown in this application, I have provided means for introducing vibrato frequencies, and this is accomplished by varying the frequency of oscillation of the twelve master oscillators periodically at vibrato frequency rates. I have also provided means for adjusting the range of the vibrato, so that the degree of effectiveness thereof may be varied to suit the character of the musical selection being rendered and to suit the individual taste of the performer.

It is thus an object of my invention to provide an improved frequency generating system for electrical musical instruments.

It is a further object of my invention to provide an improved generating system for electrical musical instruments of the piano type wherein the frequency of the generators of all notes other than the twelve notes of the highest octave of the instrument are controlled by master oscillators oscillating at the frequenclesof the twelve highest notes of the instrument.

A further object is to provide an improved electrical musical instrument having means for the generation of the necessary electrical frequencies which comprises relatively simple electrical circuits, and which may be economically manufactured.

A further object is to provide a frequency generating system for electrical musical instruments which will maintain the frequencies of oscillators over long periods of time and in which the tolerances in the electrical constants involved are relatively great.

A further object is to provide an improved frequency generating system for electrical musical instruments in which the frequency of oscillation may readily be varied to introduce a vibrato effect.

A further object of the invention is to provide an improved system of generation of the electrical frequencies required for an electrical musical instrument in which a series of generators may be readily tuned simultaneously.

Other objects will appear from the following description, reference being had to the drawings in whichFigures l and 2 together constitute a wiring diagram of representative portions of an electrical musical instrument embodying my invention; and Figure 3 constitutes a diagrammatic representation of the elements of a complete instrument showing their functional relationship.

Referring to Figure 1, tubes l0 and [2, together with their associated circuits, constitute a master oscillator which may be assumed to be that of the note A of the highest octave of the instrument and generating the frequency of 3520 cycles. Although the oscillatoris illustrated as comprising two separate tubes l0 and I2, these tubes will ordinarily be combined in a single twin triode tube. Tubes l0 and I2, together with their associated circuits, constitute an oscillator, the frequency of oscillation of which is determined primarily by the tuning circuit comprising a variable inductance It and a condenser l6 connected in parallel in the grid circuit of tube ID. The out put of tube H) is resistance coupled to the grid of tube l2 and the plate of tube i2 is connected to the grid of tube I!) by a feed-back circuit including a blocking condenser l8 and the variable resistance 20.

The signal in the output circuit of the tube 52 is transmitted through a blocking condenser 22 and coupling resistances 24, 26, and 28 to the grid 30 of a gas filled tube 32. The plate 34 of tube 32 is connected through a resistance 36 and the primary winding 38 of a transformer 40 with the positive terminal of the B current supply indicated in Fig. l as -B. The cathode 42 of the tube 32 is connected through a high resistance Ru; and a relatively lower resistance R17 to ground, to which of course the negative terminal of the B supply is also connected A condenser C15 is connected between the positive B supply and the cathode l2, and a condenser C14 is connected in parallel with the resistance R17 while a condenser C16 is optionally connectable in parallei with the resistance R11 by a manually operated switch H.

The tube 32 and the circuits associated therewith constitutes a relaxation oscillator, the constants of which are such that it will normally oscillate at a frequency less than one half that of the frequency of the master oscillator comprising the tubes l and II. If this relaxation oscillator did not have its grid connected to re-' ceive a signal from the master oscillator, its frequency of oscillation might vary slightly from its original frequency with changes in the characteristics of the tube due to aging, and with changes in temperature, humidity, and voltage of the supply. However, in view of the fact that the signal of the master oscillator is impressed upon the grid 20 of the tube 32, the frequency of oscillation of the relaxation oscillator is rigorously controlled so as to oscillate at a frequency exactly one half of that of the master oscillator. In other words,'the rate at which the condenser C is charged through the resistances R18 and R17 is such that the potential difference between the cathode and plate is not suiiicient to cause ionization of the gas in the tube until the time that the positive signal from the master oscillator is impressed upon the grid 30 of the tube 32.

The signal from the master oscillator raises the potential of the grid 30 sufficiently to cause discharge of the tube and as a result, the tube 32 is tripped by the signal from the master oscillator. It will be understood that only every other positive signal impulse received from the master oscillator and impressed upon the grid 30 is effective to cause discharge of the tube 32 because the charge on the condenser C15, and hence the potential difference between the plate and cathode, is not sufliciently great at all times that the change in potential of the grid caused by the master oscillator signal will be suflicient to trip the tube. The constants of the circuits are such that only every-other signal impulse from the master oscillator will be effective to trip the tube 32.

The discharge of the tube 32 causes a signal in the secondary 48 of the transformer 40 and the signal in the secondary is transmitted through conductor 48 and resistance 50 to the grid of the tube 32 forming part of the relaxation oscillator for the note A in the next lower octave. If the notes of a standard piano be numbered from 1 to 88 commencing with the lowest A as note No. l, the note A of the highest octave will be numbered 85 and the note A of the second highest octave will be No. 73 and the A of the third highest octave will be note No. 61. In the drawings, the oscillators for these notes are appropriately numbered and designated by the dot and dash line brackets.

The relaxation oscillator for the note Si is identical with that previously described for the note 13 except that the values of the constantsof inductance be included in the plate circuit of the tube in order to improve the form of the wave.

The secondary windings 46 of the transformers 40, and the resistance 26 may be connected to a common conductor 52 to which a switch 54 is connected to enable this conductor normally to be connected to ground, but which may, for test purposes, be connected to a terminal 5! which is maintained at a potential slightly above or below ground by a suitable potential source indicated as a battery 58. If, for example, the

The oscillators for the notes Nos. 49, 37, 25, and 7! switch 54 is connected to terminal 58 which is at a potential first of -6 volts and then +6 volts, and the tubes generate the proper signals, it will show that there is a suiiicient factor of safety in the tube circuits to assure satisfactory operation over a long period. If, upon such test, it is found that some tubes do not deliver the proper signal, appropriate adjustments will be made in the tube circuits until this desired factor of safety is obtained.

The signal from the master oscillator for the note 85 is derived from the plate circuit of the tube It! thereof through a conductor which is connected through a variable resistance 62 and a blocking condenser N, with the grid 66 of a control tube 68.

The grid 66 of control tube 68 is preferably connected to ground through a resistor 70 which may be shunted by a resistance 12 upon closure of a switch H, thereby to change the amplitude of the signal wave which is impressed upon the grid 66. The cathode 16 of the tube 68 is connected to ground through a biasing resistor '18 which is shunted by a by-pass condenser 80. The screen 82 of the tube 68 may be connected to any suitable fixed potential, such as 100 volts, while the plate 84 is connected to the output circuit through a conductor 86.

The control tube 68 has a signal from the oscillator for the note No. 85 continuously impressed upon its grid 66, but the tube 68 is normally non-conducting because of the potential which is placed upon its suppressor grid 88. The suppressor grid is adapted to be connected to ground through an attack resistor R20 by a manually operable key switch 9, and is connected to an adjustable negative potential source 9| through a high value decay resistor R11, while a condenser 52 connects the suppressor grid to ground.

Each of the gas tube oscillators 32 will supply a signal to its associated control tube 68, the signal from the gas tube oscillators being taken from a terminal 94 between the resistances Ru and R1: through a conductor 86. By closing switch 44, the shape of the wave impressed upon the grid of the control tube may be changed and hence the harmonic development of the note altered. Since the control tubes for the various oscillators are identical, only two such tubes are illustrated in the drawings, one associated with the oscillator for note No. 85 and the other associated with the oscillator for note No. 73. Howeve the plates 84 of a plurality of the control tubes are shown in Fig. 2. In general, the plates 84 of control tubes 68 are connected in parallel groups of four each. Each group of plates 84 is connected to a common conductor 88 which is connected to one terminal of the primary winding I of an output transformer I02 either through one or more resistances I04 of different values or through a condenser I06, or through-both these resistances and the condenser.

The other terminal of the primary winding I00 of the output transformer may be connected to a terminal I08 either directly, upon closure of a switch H0, or through any-desired resonant circuit branch comprising inductances H2, H3 and condensers H4, H6 and H8. These condensers may be connected to shunt the inductance H2, and condensers H5, III and H9 may be connected to shunt the inductance H3. The condensers H4 to H9 may be selectively connected in the circuit by means of individually adjustable switch arms I20, I2I which are connected between the inductances H2 and H3.

The terminal I08 which is connected to the positive B supply, is also connected through a resistance I22 to the conductor 98, the resistance I22 being adapted to baby-passed by condenser lid upon closure of a switch I26.

It will be understood that each group of plates 84 will have a circuit including resistances such as I04 and I22, and condensers, such as I06 and I24, whereas the network including the inductances II2 and H3, and condensers H4 to H9, may be common to the plate circuits of all of the control tubes 68, or, if desired, there may be two or more such networks, each network being common to a plurality of groups of plate circuits. For example, the instrument may be divided into two sections with the output for the higher pitch notes connected through one resonant circuit, while the output for the lower pitch notes is connected through another resonant circuit. In this way, the instrument may be adjusted to have two ranges of frequencies which will be accentuated. The output circuit is subject to considerable variation in design, to make possible the accentuation of certain frequencies and the at tenuation of others. The various switches by means of which the groups of plate circuits are connected to the output circuits are preferably gang switches which may be simultaneously operated manually.

The secondary of the transformer I02 is connected through a preamplifier I28 to an amplifier I30, which supplies a speaker I32.

The above described relationship of the various oscillators and resonant output circuits is 0 followed by the note number of the oscillator..

Thus, the blocks marked 0-41 to 0-88 inclusive, represent the master oscillators for the notes Nos. '77 to 88 inclusive respectively, while the blocks marked 0-I to 0-46 represent the gas tube relaxation oscillators for the notes 1 to 76 inclusive respectively. It will be noted that each oscillator representing block has an arrow leading to a smaller block representing the control tube 68 associated with the oscillator. These blocks are marked C-I to C--88 inclusive, as designating the control tubes for notes Nos. 1

be noted that oscillators for notes in octave relationship are in horizontal rows and connected by a series of arrows to indicate the fact that these oscillators are cascaded for frequency stabilization control.

Groups of the blocks representing thecontrol tube, such as C-85 to 0-88 inclusive, are connected by dash lines to a block marked RC--65-88. This latter block and the other similar blocks represent a resistive and capacitative network for each group of four notes. In Figure 2, the resistive and capacitative networks are illustrated by the resistances I04 and condensers I06.

It will be further noted that the resistivecapacitative networks arejoined by a pair of dash lines, one group of which leads to a block marked LC4I-48 indicative of the inductive capacity mesh connected to receive the signals from the control tubes for the notes 41 to iii, and the other group of which leads to a block marked LC--I-40 indicative of the inductive capacity mesh connected to receive the signals from the control tubes for notes Nos. I-40. These inductive-capacitative meshes are shown in Figure 2 as comprising the condensers Ilil to H9, and inductances H2 and H3 andassociated switches and circuits.

The signal from the blocks LC--4l-48 and LC-I--40 are indicated as being supplied to a swell control, preamplifier, power amplifier, and loud speaker, in series.

In addition, Figure 3 shows that there is a vi brato apparatus for each pair of master oscillators. Thus, for example, the block V'I688 is indicated as affecting the operation of oscillators 08'I and 0-88. The blocks V6l83, V--85 -86, etc, each represent a vibratoapparatus such as shown in Figure l, and including the vibrating reed I42 together with the means for maintaining it in oscillation, and the contacts I44 and H34 completing circuits to the tuning circuits of the oscillators.

It will be understood that the diagram of Figure 3 is not intended as a wiring diagram, but

Vibrato apparatus As previously pointed out, the frequencies of the gas tube oscillators of a cascaded series are controlled by the master oscillator, so that if thefrequency of oscillation of the master oscillator is varied at a vibrato periodicity, the frequencies of oscillation of the associated gas tube oscillators will be proportionately varied.

The means for periodically changing the frequency of oscillation of the master oscillators comprises a pair of condensers I34, I35 adapted to be connected in parallel with the condenser I6-by means of switches I36 and I31 respectively. These switches are also adapted to connect with the terminals of condensers I38, I40 which are adapted to be connected to ground through a vibrating reed I42 by means of a contact I44.

The reed I42 carries a weight I46 of magnetic material which is adapted to be attracted by an electro-magnet I48 which is energized by a battery I50 whenever the reed swings upwardly to connect contact 152 with the ground. The electro-magnet I48 thus maintains the reed I42 in a state of continuous vibration. In the complete instrument, 1 preferably provide six independently vibrating reeds I42, the weights I46 thereto 88 inclusive respectively. It will furthermore on being so proportioned that the reeds will vibrate at slightly different frequencies. Each reed will thus be arranged to control the vibrato frequency shift of two oscillators. In the drawings, the contact IE4 is a contact corresponding to the contact I44, but connected to. a different oscillator.-

By proper selection of the values of the condensers I34, I35, I38 and I49, the extent of change in frequency may be made such'that the effect upon the listener is not one of actual change of pitch, but rather one of full, rich warmth of tone. The value of the condenser I34 with respect of that of condenser I38, and that of the condenser I35 with respect to that of condenser I40 are so chosen, that the frequency change introduced by the vibrato apparatus does not result in a change in the average frequency of the note. The switches I36, I31 may be independently operated so that three different degrees of vibrato may be obtained, 1. e., (-1) when switch I" is connected to the condenser I34 and switch I31 is connected to the condenser I40, (2) when switch I36 is connected to condenser I38 and switch I3! is connected to condenser I35, and (3) when switch I36 is connected to condenser I38 and switch I31 is connected to condenser I40. When the switches are in the position that condensers I34 and I35 are in circuit, the frequency of oscillation of the oscillator will remain constant.

Since the frequency of oscillation of the master oscillator is determined not only by the condensers I6, I34, I35, I38 and I40 but also by the inductance of the coil I4, I utilize the inductance I4 as a convenient means for tuning the master oscillator, and the inductance I4 is therefore of a special construction in which the inductive reactance may be varied by small increments,

preferably by varying the amount of iron effective in its magnetic circuit. It will be apparent that the complete instrument may be tuned merely by properly tuning the master oscillators. This is a very desirable feature since the tuning operation is thus so materially simplified that it may be done by the owner of the instrument and will not require the services of an expert tuner.

Operation of frequency generating system The master oscillatoris of well-known construction and except for the addition of the vibrato apparatus, will operate in the usual manner to generate oscillations of the desired frequencies. These oscillations are impressed upon the grid 3| of the gas tube 32 for the note I3 in the system illustrated, through the blocking condenser 22 and resistances 24 and 28. The condenser C15 is being continuously charged through the resistances Ru and R11 until the potential across its terminals is sufficient to cause discharge of the tube 32. The discharge of the tube is, however, in part controlled by the potential upon the grid 34. Even though the condenser C15 may not be charged to a sufficiently high potential by itself to cause discharge of the tube 32, the presence of a positive potential upon the grid 30 will, nevertheless, cause discharge of the tube 32 through the plate circuit. Thus, the tube will discharge only when a positive signal is impressed upon the grid 34, but discharge will not take place upon each positive swing of the grid of the tube because the values of the resistances R15 and R17 with respect insufficient to cause discharge of the tube even though such discharge would be assisted by the positive potential upon the grid.

I am of course aware that gas filled triodes have been used for relaxation oscillators in various circuits where the frequency of relaxation is controlled by a signal applied to the grid of the triode. This is common practice in sweep circuits for cathode ray oscillographs. In all such circuits with which I am familiar, a relatively fixed grid bias between grid and cathode is provided with means for critically adjusting this bias so that a small signal applied to the grid will be just sufficient to cause discharge through the tube under certain circumstances. In all such circuits, the exact tube characteristics play an important role inasmuch as a deflnite grid voltage is necessary for each particular value of cathode to plate potential.

I have found in practice that there is a considerable variation between different tubes of the same lot, and serious variations which occur for the same tube due to aging, temperature and emission. These differences in tube characteristics make such circuits undesirable for an instrument of this type. I have found that the circuit disclosed herein makes unimportant the exact characteristics of the tube itself, so that the factors controlling the frequencies at which the tubes will operate can be the constants of the circuit, particularly the resistance R15 and the condenser C15. This is due to the fact that an enormous signal develops between cathode and ground across the resistances R16 and R11, and that this signal develops between grid and cathode because the grid is tied back to ground through a resistor 50. The secondary of the transformer 48 would remain at substantially ground potential except for the tripping signs. delivered by the transformer 40.

As soon as the tube ionizes, a large current flows from plate to cathode through the condenser C15, the primary 38 of the transformer 4|, and the resistance 36. Owing to the inductance of the transformer 40, the condenser C15 is not only discharged, but will receive a charge of opposite polarity so that when the heavy current flow stops, the cathode will be left at a potential more positive than the plate. At this instant, the potential of the cathode with respect to ground will be the full potential of the B supply system plus the potential developed across the condenser C15, and the grid will be left at a potential which is more negative than the cathode by this amount. Thereafter the condenser C15 starts to discharge through the resistance Ru and later begins to charge to the opposite polar- I ity.

When the next signal impulse from the oscillator one octave higher arrives, the charge on the condenser C15 will be very small so that the voltage between cathode and plate will be correspondingly small. The bias of grid to cathode will be as large as the voltage of the B battery at the time when the charge across the condenser C15 is zero. It therefore follows that the signal supplied to the grid from the oscillator one octave up canbe made very large without causing ionization of the tube for this impulse. When the next impulse comes, the condenser C15 will be charged so that the potential between cathode and plate will be large and the signal delivered to the grid from the tube one octave up will not only be sufficient to cause ionization, but would actually drive the potential of the grid positive with respect to the cathode if the tube required such a signal to cause it to ionize. Thus, the very large voltage swings which can be impressed onv the grid make the differences in characteristics of different tubes inconsequential by comparison with these high voltages.

Thus, by proper choice of values for the con densers, resistors and transformer inductances, each tube will supply the correct frequency regardless of wide variations in tube characteristics. In earlier experimental circuits, changes in tube characteristics occasionally caused tubes to ionizeat each impulse of grid signal, or to ionize for each third impulse. In that case, the note will of course suddenly play one octave up or down by the musical interval known as the fifth. With the circuit described above, all tubes of the same general type, as long as they function at all, play correctly without change in resistors or condensers and over wide variations in supply voltage, etc.

Another advantage of this circuit lies in the ability of each tube circuit to supply an independent signal with complete absence of frequencies associated with the corresponding tube one octave lower. Earlier attempts to produce a musical instrument in which independent signals were produced by frequency division were always unsuccessful, as far as I am aware, owing tc'the presence of undesired lower frequencies which are exceedingly troublesome for reasons which I will now describe.

The distort-er and control tube 68, Fig. 2, has a signal impressed upon its grid 66 which is derived from the voltage which develops across the resistor Rn, Fig. 1. This resistor Rn, which may have a value such as'10,000 ohms is in series with the resistance R16, which may be, for instance, 300,000 ohms. The resistor R11 is shunted by the condenser C14 and if this condenser C14 is omitted, the shape of the wave of voltage across R11 is that of a saw-tooth. Such a wave may be analyzed into a fundamental and a series of harmonics both of even and odd terms. The operation of the control tube, which is described in detail in my application Serial No. 199,613, filed April 2, 1938, now Patent 2,126,464, granted August 9, 1938, is such that only the most positive excursions of grid voltage are effective to produce a plate current which flows in impulses in order to produce a tone very rich in harmonics. 1

If the signal which develops across the resistor Rn contains even very small amounts of frequencies lower than the fundamental frequency associated with that note, or if it contains any odd harmonics of a lower frequency such as odd harmonics of the fundamental of the octave down, then sum and difference terms will arise in this non-linear circuit which will completely spoil the musical effect of the note. Now the gas filled tube 32 associated with note No. '73, in addition to supplying a signal for that note, must operate to trip the gas tube one octave down associated with note No. 61. When this second tube ionizes, there is an opportunity for a signal of lower frequency to be reflected back into the tube of note No. 73 through capacity effects within the tube of note No. 61, the plate of which exhibits violent voltage changes at a frequency an octave lower. It is therefore necessary to prevent reflections from this source from entering the signal supplied by the tube of note No. 73. This is done by means of the circuit shown and the proper choice of the constants short period of time of discharge.

of the circuit. Thus, a resistance 50, Fig. 2, which may have a value of 250,000 ohms is inserted in series with the grid connection to the driven tube so as to limit to a small value the current which may flow in this circuit upon ionization of the tube. The signal to drive the tube 32 for note 6| is derived from the secondary of transformer 40, Fig. 1, this secondary 46 having preferably many less turns than the primary 38 so as to reflect the smallest possible load into the driving circuit including the primary 38, the resistance 36, the plate-to-cathode circuit through the tube, and the condenser'Cus. The resistance 36 may have a value of 2,000 ohms. It will be noted that the signal for note No. 73 is not taken from thiscircuit, so as to further avoid the possibility of feed-back. The condenser- C15 is charged at a relatively slow rate through the resistances Ru and R11 from the B voltage supply. When ionization takes place in the tube, the condenser discharges at an exceedingly rapid rate through the cathode plate circuit just described, which is of very low impedance. The only feed-back from the lower note which could occur is during the exceedingly Thereafter the plate circuit ceases to have any effect on the condenser (315 which again starts to charge through the resistances R18 and R11. By this arrang'ement the effect of the driven tube is prevented from causing reflection into the driver, and the use of additional intermediary tubes for preventing reflection is avoided.

The resistance 36 and inductance 38 cause the wave of voltage between the cathode l2 and ground to assume the form of a saw-tooth wave having slightly rounded peaks. The signal from the gas tube oscillator for the note 13 is derived from the terminal 94 and is likewise of generally saw-tooth wave form, and is impressed upon the grid of the associated control tube 68 through conductor 96. The negative potential impressed upon the suppressor grid 88 through the resistance R21 is normally sufficient to prevent any plate current in the tube 68. When, however, it is desired to sound the note 13, the key switch 90 associated therewith is depressed, thus connecting the suppressor grid 88 to ground through a resistor R20.

Upon initial closure of this switch 90, the condenser 92 is discharged through the resistor R20 and the potential upon the suppressor grid 88 will therefore be changed to ground potential gradually instead of instantaneously. This is a desirable feature since the corresponding gradual increase in plate current will cause the note to be sounded with a gradual attack instead of with a sudden abrupt attack. Since the resistor R20 is of much lower value than resistor R21, the suppressor grid will shortly attain ground potential, with a resultant flow of plate current.

Upon release of the key, the potential of the suppressor grid will not immediately return to its normal negative potential but, due to the discharged condition of the condenser 92 at this time and the relatively high resistance of the resistor E21,. the drop in potential of the suppressor grid will be gradual at a logarithmic rate. The resultant gradual decay in the plate current and consequent decay in the note being sounded is very advantageous since it provides the effect of reverberation in the music being rendered. The instrument may thus be played in a room which is acoustically relatively dead" without disadvantage.

The form of the saw-tooth wave impressed upon the grid of the control tube It may be altered to have its positive peak made more rounded by closing the switch 44 and thus connecting the condenser Cu in parallel with the condenser C14. The wave thus impressed upon the grid of the control tube may thereby be controlled so as to have different degrees of harmonic development, it being understood that the wave having the sharper peaks will cause distribution of the greater proportion of its energy in the higher harmonics with a resultant brilliancy or keenness in the sound produced. These effects are more fully discribed in my application Serial No. 199,614, flied April 2, 1938, now Patent 2,126,682, granted August 9, 1938.

By means of the various gang switches associated with the resistors I04 and condenser I06, as well as the network including the inductive impedances H2 and H3 and condensers ill to H9, and associated switches I20, HI, and I25, the frequency of resonance may be varied, and the quality of tone of the instrument as a whole may be varied.

The system may be provided with a volume control diagrammatically illustrated in Fig. 2,

comprising a variable resistance I52 and condenser l5l connected across the secondary winding of transformer I02.

The provision of the vibrato apparatus and the adjustable resonance output circuits makes it possible to produce music of pronounced stringlike quality simulating that produced by instruments of the violin family, with the pronounced advantage that one musician can, by

means of the instrument, simulate the effects produced by a string quartet, or the like, since of course the musician can play the several parts with the instrument of the present invention because it will be provided witha keyboard similar to that of the standard piano or organ.

While I have shown and described a particular embodiment of my invention, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the essential features of the invention.

For example, the master oscillators may be replaced by any other suitable group of generators of electrical'impulses; different types of output and key controlled circuits may be provided for transmitting the electrical impulses of the generators to the electro-acoustic translating means; and various forms of frequency controlled oscillators or frequency dividing circuits may be utilized in various combinations. I therefore desire to include within the scope of my invention, as defined by the following claims, all such similar constructions whereby substantially the same results may be obtained by subtantially the same or equivalent means.

I claim:

1. In an electrical musical instrument, "the combination of a plurality of master oscillators each having a tuned reactance mesh controlling its frequency of oscillation, a plurality of means for periodically changing the reactances of the meshes of a plurality of said oscillators at slightly different vibrato frequency rates, and a plurality of cascaded series of generators, each series having its frequencies controlled by one of said master oscillators, whereby changes in frequency of oscillation of said master oscillators will cause proportionate changes in frequencies of their associated series of cascaded oscillator.

2. In an electrical musical instrument, the

combination of a source of electrical impulses of accurately maintained musical frequency, a relaxation oscillator for producing impulses of one half the'frequency of said source, said relaxation oscillator comprising a gas filled multi-electrode thermionic tube having a cathode, grid and plate; a circuit connecting the grid of said tube with said source, a cathode-plate circuit including a source of direct current potential having its positive terminal connected to the plate of said tube through an inductance and a resistance element, and having its negative terminal connected to said cathode through a relatively high value resistor; a condenser connected between said cathode and the positive terminal of said direct current potential source, and means connected across said resistor for deriving a signal from said oscillator.

3. In an electrical musical instrument, the combination of a source of electrical impulses of fundamental frequency of a note of the musical scale, electro-acoustic translating means, and means for controlling the transmission of the signal generated by said source to said translating means, said control means comprising an electron discharge device having a cathode, plate and at least two grids, a circuit for impressing a signal from said source upon one of said grids, a direct current potential source connected to the second of said grids normally to prevent said tube from transmitting said signal, and key controlled switch means for changing the potential on said second grid in a manner to render said tube capable of transmitting said signal.

4. In an electrical musical instrument, the combination of a source of electrical impulses of accurately maintained musical frequency, and a relaxation oscillator for producing impulses of exactly one half the frequency thus produced by said source, said relaxation oscillator comprising a gas filled multi-electrode thermionic tube having a cathode, grid and plate; means for impressing impulses from said source upon the grid of said tube; a cathode-plate circuit including an inductance, a source of direct current potential and a high resistance in series; and a condenser connected between the cathode of said' tube and a positive terminal of said direct current potential source, said condenser being charged from said source through said high resistance and discharged through said cathode plate circuit of said tube.

5. In an electrical musical instrument, the

combination of a plurality of sources of electrical impulses of frequencies of the musical scale, a pentode control tube for each of said sources and having its control grid connected to receive impulses from its associated source, an output circuit connected in the plate circuit of said control tube, and key controlled means for changing the potential on the suppressor grid of said control tube thereby to render said tube capable of transmitting to said output circuit the signal impressed upon its grid.

6. A relaxation oscillator for generating a signal having a frequency of a note of the musical scale comprising, a gas filled tube having an anode, a cathode, and a. grid; a source of plate current; an input circuit for said tube having means for impressing impulses of twice. the frequency of said note upon the grid of said tube; a high resistance and an inductance connected in series with said source between said cathode and said plate; and a condenser connected between the positive terminal of said source and said cathode.

7. The combination set forth in claim 6 in which said high resistance is connected between the negative terminal of said source and said cathode.

8. In an electrical musical instrument having a plurality of groups of generators, each generator producing electrical impulses of one of the notes of the musical scale, the combination of, a plurality of vibrato frequency devices, and means for connecting each of said devices to a group of said generators for varying the frequencies thereof, whereby the vibrato on different notes may be made to differ in phase and frequency.

9. The combination set forth in claim 8 in which each of said vibrato devices comprises a vibratory element determining its frequency, and means for maintaining said element in vibration.

10. In an electrical musical instrument having a master oscillator, means for varying the frequency of oscillation thereof to produce a vibrato effect, comprising a tuning element for note ofthe musical scale and each series generating frequencies of notes in octave relation ship, and having twelve tunable master oscillators, one for each series of cascaded generators, the combination of a plurality of members moving at slightly different vibrato frequency rates,

a plurality of switches operated by said members, and means including said switches for changing the tuning of the master oscillators,

13. In an electrical musical instrument having a master oscillator and means for varying the frequency of oscillation thereof to produce a vibrato effect, said means comprising a tunable element for said oscillator, the combination of a member moved continuously at a vibrato frequency rate, and means operated by said member said oscillator, a member capable of vibration at a vibrato frequency rate, electro-magnetic means for vibrating said member, and a switch operated by said member to render said tuning element alternately effective and ineffective, thereby to change the frequency of oscillation of said oscillator.

11. In an electrical musical instrument having a master oscillator for controlling the frequency of oscillation of a plurality of cascaded generators, means for introducing a, vibrato change in frequency in the impulses produced by said master oscillator and its controlled generators comprising, a reactance mesh determining the frequency of oscillation of said master oscillator, a member moving at a vibrato frequency rate, a circuit including means for changing the reactance of said reactance mesh, and a switch operated by said member to open and close .said circuit at a vibrato frequency rate.

12. In an electrical musical instrument having twelve cascaded series of generators, each generating the fundamental frequency of a different to render such tuning element alternately eifec' tive and ineffective thereby to change the frequency of oscillation of said oscillator at said vibrato frequency rate.

14-. In an electrical musical instrument having a master oscillator for controlling the frequency of oscillation of a plurality of cascaded generators, means for introducing a vibrato change in frequency in the impulses produced by said master oscillator comprising a reactance mesh including a capacitance determining the frequency of oscillation of said master oscillator,

a member moving at a vibrato frequency rate, a

circuit including said capacitance for changing the'reactance of said reactance mesh, and a switch operated by said member to open and close said circuit at said vibrato frequency rate.

15. In an electrical musical instrument having a plurality of cascaded series of generators, each generating impulses of a frequency of a different note of the musical scale and each series generating frequencies of notes in octave relationship and having a plurality of master oscillators, one for each of said series of cascaded generators, the combination of a plurality of members moving at slightly different vibrato frequency rates, a plurality of switches operated by said members, and means controlled by each of said switches for changing the tuning of one of the master oscillators.

LAURENS HAMMOND. 

